Polyglutamine (PolyQ) diseases are a class of diseases consisting of nine genetically distinct disorders. They include Huntington's disease (HD), dentatorubral-pallidoluysian astrophy (DRPLA), SBMA and spino-cerebellar ataxia 1, 2, 3, 6, 7 and 17 (SCA1/2/3/6/7/17). Because these diseases are caused by the expansion of a translated CAG repeats that codes for the glutamines, they are also known as CAG repeat diseases.
One common physiological characteristic shared among these genetically distinct diseases is that patients who suffer from the diseases are all found to have proteinaceous deposits in their brains. Although in each of these diseases, the proteinaceous deposit is associated with a different protein, the proteins all contain an expanded stretch of glutamines. To date, this expanded stretch of polyQ sequence in the disease-related proteins is the only known genetic mutation implicated in all the polyQ diseases.
In general, the number of CAG repeats in genes can range from a benign number of less than 36 to a pathological number of 37 or more. The larger number of CAG repeats are thought to correlate to pathological phenotypes because proteins and polypeptides that contain a long stretch of glutamines have an inherit propensity to form amyloid-like fibrils (polymerization of protein aggregates with β-sheet structure) in vitro (Scherzinger et al., 1997), and mutant proteins with an expanded polyQ tract are thought to result in a distinct protein conformation that leads to aggregation and eventual neuronal cell death (Zoghbi and Orr, 2000).
In human, expanded polyQ mutant proteins are expressed widely in cells of the central nervous system (CNS), however, in each different disease, a specific population of neurons is more vulnerable than others. Consequently, the difference in vulnerability results in characteristic patterns of neurodegeneration and clinical features for each of the nine different diseases. The severity of the disease may correlate to the number of CAG repeats. For example, in HD, CAG repeat numbers between 28-35 are considered to be intermediate, 35-40 are considered reduced penetrance, and repeat numbers greater than 40 are considered to be full penetrance.
Table 1 lists eight diseases caused by the expanded CAG repeats, the affected genes, and their defining pathogenic repeat length. SCA6 is not included in this list because unlike other polyQ diseases, the length of CAG repeat in SCA6 is not a determining factor for the age that symptoms begin to present. Pathological repeat length in SCA6 is also much shorter than the other polyQ diseases, where a number between 21-30 is sufficient to cause pathological phenotype.
TABLE 1Gene name/PathogenicDiseaseprotein productrepeat lengthSpinocerebellar ataxia type 1SCA1SCA1/ataxin 140~82Spinocerebellar ataxia type 2SCA2SCA2/ataxin 2 32~200Spinocerebellar ataxia type 3SCA3SCA3/ataxin 361~84(MJD)Spinocerebellar ataxia type 7SCA7SCA7/ataxin 7 37~306Spinocerebellar ataxia typeSCA17SCA17/TBP47~6317Dentatorubral pallidoluysianDRPLADRPLA/49~88atrophyatrophin 1Spinal and bular muscularSBMAAR/androgen38~62atrophyreceptorHuntington's diseaseHDHtt/huntingtin 40~121
Of the above eight diseases, HD is perhaps the most well-known among the general public because of its devastating effects on the patients. The disease is associated with selective neuronal cell death occurring primarily in the cortex and striatum. It is a fatal and cruel disease that progressively deprives the patient of his movement, cognition, and personality, exacting significant economic and emotion tolls on the patient and his family. The frequency of HD is particularly prevalent among people of Western European descent (about 1 in 20,000). Unfortunately, there is presently no cure for this terrible disease.
Currently, available treatments for HD are mainly limited to managing the macroscopic symptoms. For example, one of the newest compound approved by the FDA, tetrabenazine, is a drug for reducing hyperkinetic movements in HD patients. Tetrabenazine is a vesicular monoamine transporter (VMAT) inhibitor which promotes early degradation of neurotransmitters. Thus, the drug merely treats the symptom, not the root of the disease. Other drugs currently used for treating HD include neuroleptics and benzodiazepines. As the disease progresses, an ever wider range of pharmacopeia is needed to address different symptoms, including antipsychotics, and drugs for hypokinesia. No presently known treatment is attempting to address the root cause of HD.
As mentioned above, the root cause of HD is an abnormal expansion of CAG repeats in a gene within the CNS cells, specifically the gene Htt which encodes the protein huntingtin (Htt). In a normal person, there are about 8-25 constitutive repeats of CAG nucleotide sequence in the Htt gene. In a HD patient, the number of CAG repeats are expanded to 36 or more. Because this type of mutation is dominant, a person only needs to inherit one copy of the mutated huntingtin gene to develop HD.
Recent cell and animal model studies have shown that aggregates formed by mutant Htt play a critical role in the progression of HD. It has been observed that the mutant Htt proteins can leave behind shorter fragments from parts of the polyQ expansion when subjected to proteolytic cleavages. If too many copies of glutamine exist in the mutant Htt, the polar nature of glutamine will lead to undesirable interactions with other proteins. In particular, mutant Htt with too many copies of glutamines will form hydrogen bonds with one another and aggregate rather than fold into functional proteins. Over time, the accumulated protein aggregates will damage the neuronal cells, leading to cell death and neurological deficit in the patient. The damaging effects of the protein aggregates have been corroborated by experiments showing that chemical reagents capable of inhibiting the formation of protein aggregates can enhance survival of cells and ameliorate pathology of HD in a mouse model (Sanchez et al., 2003; Tanaka et al., 2004).
Besides using inhibitory molecules to prevent protein aggregation, reducing the expression of mutant huntingtin gene is in principle an alternative way to inhibit the genesis of insoluble protein aggregates. In vitro studies have shown that the extent of polyQ protein aggregation is related to protein concentration (Scherzinger et al., 1999). Therefore, by lowering the level of mutant huntingtin gene expression, a lower level of expanded PolyQ protein will be expressed, which in turn is likely to reduce protein aggregate formation and delay the onset of HD.
These findings point to a potentially simple and powerful strategy of combatting HD pathogenesis by modulating the formation of insoluble protein aggregates resulting from CAG repeat mutation in Htt. For example, a therapeutic agent that can modulate the expression of the polyQ mutant genes or formation of the polyQ aggregates can potentially address the root cause of the polyQ diseases, not just their physiological symptoms. Unfortunately, the lack of knowledge about cellular factors and agents that can modulate the expression of the mutant polyQ genes has prevented practical development of this therapeutic strategy.
Therefore, there still exists urgent needs for methods and tools that can modulate or reduce the expression of genes suffering from expanded CAG repeat mutations as well as methods and tools for identifying and developing agents that are effective at modulating or reducing the expression of the mutant genes.